The inmate population at one of North Korea's most notorious prison camps, Camp 16, is expanding, according to an Amnesty International report detailing rape and torture at the facility.

The report by the London-based rights watchdog includes rare testimony from a former camp guard, as well as from former inmates about the brutality prevalent in the prison system.

North Korea denies the existence of the political prison camps which, according to independent estimates, form a network holding between 100,000 and 200,000 people.

Satellite images analysed in the report estimate that Camp 16, an infamous gulag, is 560 square kilometres with around 20,000 prisoners.The images indicate a slight increase in the remote camp's population, with new housing blocks clearly visible and signs of "significant" economic activity such as mining and logging, the report said.